Recent drop in kH

[zdrc]

I have a 12 gallon aquapod that's been set up for a month and a week (first saltwater tank). I bought some KP aquatics live rock and the tank cycled in about two weeks. I've been testing the water weekly when I do water changes (usually ~25% weekly water changes). The tests are all Salifert and I use a glass hydrometer calibrated to 60F. The hydrometer reads specific gravity about 0.002 below the actual specific gravity at 78F (I've checked this with a refractometer at a LFS). I'll show you my typical water parameters with a mean and standard deviation and the most recent measurement I've made. Note, there is a two week gap between these most recent tests and the previous tests. I did do a water change last week though I was unable to test the water thoroughly (just salinity). Salt is Fritz RPM blue box.

TYPICAL WATER PARAMETERS
Specific gravity: 1.0242 +/- 0.0003 @ 60F (this should be ~35 ppt according to LFS refractometer)
kH: 8.3 +/- 0.2
Ca: 402 +/- 7 ppm
Mg: 1450 +/- 30 ppm
pH: 8.05 +/- 0.10
NO3: 7.5 +/ 2.5 ppm
PO4: <0.03 ppm (never read more than this)

For reference, an ICP test I ran about a month ago said that my Ca was 428 ppm and Mg was 1237 ppm which are both completely outside the range of values that my testing has found.

MOST RECENT MEASUREMENT (day after 25% water change)
Specific gravity: 1.024
kH: 6.7-7.0 <---!!!
Ca: 390 ppm
Mg: 1410 ppm
pH: 8.0-8.15
NO3: 5 ppm
PO4: <0.03 ppm

This kH reading is abnormal and consistent (I tested twice). All other readings have occurred before (I've gotten 390 ppm Ca and 1410 ppm Mg in the past). A little over two weeks ago I got some CUC (2 trochus, 2 cerith, 1 nassarius, and 1 scarlet hermit) and my first two corals, a Ricordea mushroom and a neon green Caulastrea frag. The Caulastrea frag looks great, I'm getting some pretty awesome polyp extension. The Ricordea is so-so. It came loose, and I'm having a very hard time getting to attach to anything. I've put it in a little cup with some rubble, but it's been a week and it hasn't attached to anything. I think I should just leave it be for now though. The coralline algae that came with the rock is going ham, it seems to be plating right off the rock and forming little chalices. This could be a kH sink? It's interesting that all the other parameters are more or less normal and the kH has changed so dramatically.

I'm pretty sure I need to increase my kH a bit. I'm planning on bumping the salinity up to 1.0245 next water change, but I think I need to add something to increase kH a bit, maybe 0.5 every few days until I'm back over 8.0. What should I use to increase kH? Kalkwasser?

 

[blaxsun]

Yes, coralline growth can be a bit of a sink for alkalinity and magnesium. That and the corals will account for a bit - but not that much.

I suspect it may have more to do with your last 25% water change - especially if the salinity was off. I'd get a refractometer and use that - you may find your salinity is a lot lower than 1.024.

 

[zdrc]

Yes, coralline growth can be a bit of a sink for alkalinity and magnesium. That and the corals will account for a bit - but not that much.

I suspect it may have more to do with your last 25% water change - especially if the salinity was off. I'd get a refractometer and use that - you may find your salinity is a lot lower than 1.024.

This is possible, I can take another water sample to get the salinity checked. It is unusual because I checked the salinity of the tank and the water change bucket before making the water change, and they were in agreement. I then checked the water after the change an it was a little higher than I wanted, almost 1.025, and I did add a cup of unsalted water to get it back down by 1.024. I account this to the difficulty of measuring salinity in the bucket with the glass hydrometer. I checked the salinity a day later and it was consistent with the test last night. Glass hydrometers like mine aren't really supposed to lose calibration, and I did already take some 1.024 water to my LFS to verify that it is 1.026.

I will measure salinity again just to make sure I didn't make any mistakes when using the hydrometer.